Magnetic disks are used in computer systems as the primary means of storing data. Conventional methods of data storage on disks use the process of longitudinal magnetic recording. Disks used with such a process consist of a layer of a high coercivity `hard` magnetic layer, such as a cobalt based alloy, that is directly deposited onto a conductive substrate base. A more recent method utilized to increase the storage density of magnetic disks uses perpendicular or vertical magnetic recording. The use of such process requires a film of low coercivity `soft` magnetic material, such as permalloy a Nickel--Iron (NiFe) alloy, to be deposited onto a disk substrate. Over this permalloy layer is deposited a vertically or perpendicularly oriented `hard` magnetic data storage layer that can be magnetically influenced to record information, commonly encoded in digital (binary) form. The permalloy layer effectively functions as a part of the recording head beneath the vertically oriented hard magnetic layer, providing a magnetic return path which decreases the magnetic reluctance for the head. The coating of permalloy magnetic material and the hard magnetic material on the disk substrate is often done by the process of electroplating or electrodeposition.
The distribution of permalloy magnetic material should be of uniform thickness over the entire surface of the disk substrate. This is necessary in order to meet minimum plating thickness requirements, to reduce post-plating surface finishing activities and to attain high quality information recording at low noise levels. Further, by achieving uniform coating thickness, the amount of material that has to be removed by post-plating surface finishing processes is reduced, thereby minimizing the total amount of plated material consumed. Commonly used methods of cathode robbing or thieving for removing excess plated material are inefficient. Then too, by ensuring a uniform thickness of plated material on a disk substrate surface, surface flatness is achieved and the surface flatness of disk substrates is a key performance criteria. A flat surface results in efficient functioning of the disk substrate and head assembly by minimizing the mechanical acceleration forces required for the head to follow the disk as it spins.
In order to achieve uniform plating distribution there must exist uniform current distribution at the surface of the disk substrate during electroplating. Prior art processes have not been very effective in controlling plating uniformity over the entire surface, especially at the outer and inner edges, of the disk substrate. Accordingly, there always exists a need for an apparatus that ensures the establishment of uniform current distribution across the entire surface of the disk substrate, to facilitate the uniform deposition of magnetic material during electroplating.
The permalloy magnetic material must also be magnetically oriented, in generally the same preferred circumferential direction, when deposited on the surface of the disk substrate for optimum disk performance. The magnetic orientation of the deposit results in greater magnetic permeability (permanence ratios &gt;2.0) of the deposit in the preferred circumferential direction compared to the radial direction. Such a preferentially oriented magnetic deposit is less sensitive to stray magnetic fields and therefore produces less noise in the recording system.
Therefore there must be a source of magnetic flux to orient the magnetic material at the time of deposition of the coating material on the disk substrate. Prior art electroplating processes have used large electromagnets or large permanent magnets, placed outside the plating tank, as a source for magnetic flux for the orientation of the particles. U.S. Pat. No. 3,141,837, issued to Edelman, discloses one of the prior art methods for electrodepositing nickel iron alloys on a substrate. The Edelman method uses a permanent magnet positioned around the outside of the tank to provide an orienting magnetic field to the alloy to be electrodeposited. Due to the large size and the distant positioning of the magnets, the previous processes have not been able to provide small localized areas of magnetic flux. This results in only a limited number of disk substrates that can be electroplated at any one time while achieving both acceptable plating uniformity and magnetic orientation. Moreover, the prior art methods, as described above, are relatively expensive, due to the size of the magnets. Also, due to the inefficient conductance of flux energy, existing substrate plating systems have the capability of plating only a few substrates at a time. U.S. Pat. No. 4,720,329, issued to Sirbola being an example of one such method. Therefore, there exists a need for cheaper and more efficient and effective electroplating processes for disk substrates, suitable for mass commercial production.
The previous processes also experienced the problem of "plate-up" of substrate holders. The substrate holders have to be stripped to remove the plated material before re-use thereby making the process costly and inefficient.
The electroplating process deposits magnetic material on the disk substrate surface by the reduction of metal ions with electrons at the disk substrate surface. This results in ion depletion in the electroplating solution in the immediate vicinity of the disk substrate. Ion depletion leads to a non-uniform electroplating rate, causing both non-uniform plating thickness and non-uniform concentration of ions in the deposited magnetic material. Ion depletion can be corrected by replenishing ions at the cathode surface during electroplating by the mass transport of ions to the disk substrate surface, using mechanical agitation methods to stir up the electroplating liquid. The commonly used `knife-edge` methods of horizontal or vertical motion of disk substrates results in the preferential replenishment of ions and thereby non-uniform plating along the leading edges of the substrate, perpendicular to the direction of travel.
Also, generally fixed magnets are used to align the deposited magnetic material. However, by using a vertical or horizontal `knife-edge` agitation method for moving the disk substrates, the radial magnetic field cannot be maintained when fixed magnets are used. As a result, the orientation of the deposited magnetic material tend to be uniform in the direction of `knife-edge` movement, but variable in the perpendicular direction.